I will follow up on Valery's comments. We use these words together like this in the US: 1. Devoted to 2. Spend on She is devoted to her husband. He would spend all of his money on candy. #1 sentence above would read: You must devote more time to your English #2 sentence is correct.

"devote" - expresses the idea of looking for time for something. "spend" - expresses the idea of putting some part of your time on something. Something like this.

To be quite honest, and I may be incorrect, but I don't think there is a difference between these two sentences. One thing to note is that you would write the first part as "You must devote morе tіmе to your Еnglіsh" or "You must devote morе tіmе to learning Еnglіsh".